I used to have a lot of mail, dating back to 2006 in Webmail. However, only mail from the past seven days seems to be kept at the website, using Firefox 3.5.7 or Seamonkey2.x, or IE8.
All my Sent mail is still there. I don't see any option pertaining to the number of messages left in the Inbox, nor a length of time.
Do you know if Yahoo made some changes?
UserAgent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:220.127.116.11) Gecko/20091221 Firefox/3.5.7